LTE (Long-term evolution) is a project within the 3GPP (3rd Generation Partnership Project) with an aim to improve the UMTS (Universal Mobile Telecommunications System) mobile phone standard for coping with future technology evolutions. The LTE comprises developing a new air interface standard, and the downlink (base station to user equipment) will be based on OFDMA (orthogonal frequency division multiple access). For the uplink (user equipment to base station), SC-FDMA (single carrier frequency division multiple access) is an attractive choice as SC-FDMA has a lower peak-to-average power ratio than OFDM. The lower peak-to-average power ratio entails improved transmitter power efficiency for the battery-operated user equipment, which is an important design consideration.
In any wireless communication system, a transmitted signal is distorted due to dynamic properties of a radio channel through which it is transmitted. In order to compensate for the dynamic properties of the radio channel, different methods are available for combating interference. An ideal compensation would completely cancel the effects of the radio channel and the resulting equalized channel would be completely frequency flat. However, such a scheme would in most cases lead to unwanted noise amplification limiting the performance. Equalization schemes must therefore provide a trade-off between noise amplification and making the equalized channel frequency-flat.
For the transmitted data to be recovered at the receiver it is important that the interference is suppressed. Besides the mentioned power consumption aspect of the user equipment, there is also a desire to restrict the size and costs of the user equipment in order for it to be attractive. The desire to reduce size, cost and power consumption is valid also for receivers in the base station. The space for and costs of processing circuitry should therefore be kept at a minimum. The complexity of the methods used for combating the interference competes with a desire to cancel the interference to as large extent as possible. The designer thus stands before the choice of using interference combating algorithms having less than optimal performance and designing a rather complex and consequently expensive receiver. In short, there is a trade-off between complexity of receiver and performance in terms of accuracy.
A particular example of this trade-off is the choice of decoding scheme to be used in the receiver. An advanced detection scheme is the maximum likelihood detection (MLD), but it has a computational complexity that is exponential with the number of modulation symbols. Efforts have been made to reduce the computational complexity to acceptable levels, an efficient implementation of MLD being, for example, sphere decoding.
In view of the above, it would be desirable to provide simplified and yet effective interference cancellation methods, and in particular a MLD having even further reduced complexity than the hitherto known methods.